I. Field of the Invention
The present invention relates to a diagnostic system for detecting leaks in the evaporative emission control system of the type used in automotive vehicles.
II. Description of the Prior Art
Government regulations require that automotive vehicles be equipped with an evaporative emission control system in order to prevent, or at least minimize, the release of volatile fuel vapors to the atmosphere. Such systems typically comprise a carbon filled canister having both a vent port and a fuel vapor port. The fuel vapor port is connected to a vapor line open to the fuel tank head space so that fuel vapors from the fuel tank are adsorbed by the carbon in the canister.
Additionally, the canister vapor port together with the vapor line from the fuel tank are connected through a purge valve to the intake manifold of the engine. The purge valve is typically solenoid operated and is controlled by the on board computer for the engine. In operation, during certain engine conditions which are preprogrammed into the on board computer, the on board computer opens the purge valve and allows the vacuum from the intake manifold to induct vapors which have previously been adsorbed by the carbon in the carbon canister. Such inducted vapors are, of course, combusted in the engine in the desired fashion.
U.S. governmental regulations require that certain future automobiles be equipped with on board diagnostic capability for determining if a leak is present in the evaporative emission control system for the vehicle. In particular, leaks in the evaporative emission control system greater than a preset amount, for example 0.04 inches in diameter, must be detected and reported to the vehicle operator.
There are a number of previously known proposed systems for detecting leaks in the evaporative emission control system. These previously known systems, however, require relatively expensive additional engine components which unduly increase the overall cost of the leak diagnostic system.